Question: In physics class, the girl to boy ratio is $10$ to $5$. If there are $10$ boys, how many girls are there?
Answer: A ratio of $10$ girls to $5$ boys means that there is a group of $10$ girls for every group of $5$ boys. If there are $10$ boys, then there are $2$ groups of $5$ boys each. Because we know the ratio is $10:5$ , there must be $2$ groups of $10$ girls each. There is a total of $20$ girls in physics class.